uipc_shm: Implements fspacectl(2) support

This implements fspacectl(2) support on shared memory objects. The
semantic of SPACECTL_DEALLOC is equivalent to clearing the backing
store and free the pages within the affected range. If the call
succeeds, subsequent reads on the affected range return all zero.

tests/sys/posixshm/posixshm_tests.c is expanded to include a
fspacectl(2) functional test.

Sponsored by: The FreeBSD Foundation
Reviewed by: kevans, kib
Differential Revision: https://reviews.freebsd.org/D31490